Most existing assays for evaluating drug toxicity are relatively expensive and time-consuming. One of the most powerful fluorescent assays is applicable only for targeted cellular structures that are themselves fluorescent or can be labeled with fluorescent markers. The goal of this proposal is to develop a new, highly sensitive photothermal (PT) assay for rapid detection of nonspecific drug toxicity at the live single-cell level by the monitoring of drug-induced alteration in light-absorbing, nonfluorescent endogenous cellular structures that can been used as natural markers. It is assumed that drug action can be mediated through various biochemical processes and cellular targets of light-absorbing components of the respiratory chain. The Specific Aims of this proposal are as follows: Aim 1: Discover the toxicity-sensitive parameters of PT assays in different modes at the single-cell level. Aim 2: Validate the new PT assay for cytotoxicologic screening of antitumor drugs. Development of an innovative PT assay is important both for basic biologic studies of cell metabolism and for applied pharmaceutical research. The potential advantages of a new assay in comparison with existing assays include highly sensitive control of a single cell in its native state, no chemical pretreatment of cells, relatively low cost, significant improvement in test time, and ecologic safety. The long-term goals of this proposal include the development of new modifications of the PT assay for initial estimation of the efficacy of therapeutic cancer agents, study of phototoxicity and embryotoxicity, and improvement of spatial resolution to study drugs' toxic effects at the DNA level.